[~TASK] FLOW3 (Core): The Bootstrap will now accept only "Production" or "Development" as valid contexts because this is how we currently use FLOW3. As soon as we really now how we want to support additional contexts, we can make it more flexible again. As a side effect, FLOW3 won't complain about missing context directories in the global Configuration dir.
[~TASK] FLOW3 (Security): Set a default encryption key for the time being until automatic generation is implemented. Relates to #7516